In a conventional nursing bottle having rigid side walls, both formula and air are present such that both are
by a feeding infant which feels discomfort and distress due to the presence of air in the infant's stomach. The development of nursing bottles comprising thin-walled shells that enclose a flexible liner that functions as a container per se was intended to alleviate if not eliminate the problem of air ingestion.
It was believed that as formula was drawn from the flexible liner, the side walls of the liner would collapse about the remaining formula, thereby avoiding the creation of a partial vacuum within the bottle as occurs in those bottles having rigid sides. It has been determined, however, that air will still be drawn into a nursing bottle having a flexible liner, especially after the contents are partially drawn out and the bottle is turned upright. Although the side walls of the liner do collapse, the tension exerted on the side walls by the weight of the formula at the bottom of the liner tends to separate the collapsed side walls which generates a partial vacuum within the liner that draws in air.
As a result of the continuing problem, various forms of pushsticks have been developed to expel air from nursing bottles having flexible liners.
A typical example of apparatus for expelling air from a nursing bottle is illustrated and described in U.S. Pat. No. 3,648,873 Grobbel. The apparatus includes a base upon which a dowel is vertically mounted, the uppermost end of the dowel having attached thereto a short cylindrical member that is slidably reciprocable within the bottle shell to collapse the liner for expelling air therefrom.
Another example of similar apparatus is described in U.S. Pat. No. 4,176,754 Miller. Instead of a fixed combination of a short cylinder attached to the end of a dowel, Miller describes and illustrates a soft wheel that is rotatably mounted at one end of a handle. The diameter of the wheel is somewhat greater than the inner diameter of the bottle's shell so that when the wheel is inserted into the shell through an open end, the wheel is compressed and a inner is expelled by compressing the liner by means of the wheel. However, the restoring force set up within the wheel maintains the wheel in any given position within the bottle to ensure that air is not subsequently drawn back into the liner.
Both Grobbel and Miller describe structures that, while eminently suited to the task of expelling air from the flexible liner, are large and relatively bulky such that these structures do not appear to be cost effective. Moreover, the relatively large size of both structures presents a storage problem which is further aggravated if more than one is used in a household.